1. Field of the Invention
This invention relates to an improved silent discharge type of gas laser apparatus.
2. Description of the Prior Art
FIG. 1 shows a vertical cross-sectional view of a conventional silent discharge type gas laser apparatus, in which the gas flows in parallel with an axis of the laser radiation. The operation of this laser apparatus will be described using a CO.sub.2 gas laser as an example.
A gas mixture of CO.sub.2, N.sub.2 and He is fed into a discharge tube 1 made of dielectric materials at a pressure of 100 torr and is circulated in the direction of arrows A by a blower 9. A signal having a frequency from 20 or 30 KHz to several hundred KHz and a voltage of several KV is applied across electrodes 2-1 and 2-2 from a power source 4 to form a discharge in the tube 1. This discharge takes place between the two electrodes disposed on the dielectric wall of the tube, and is called "silent discharge".
The CO.sub.2 molecules in the gas are excited by the discharge and laser oscillation is established in a resonator including a total reflector 5 and a partial reflector 6; laser radiation is primarily emitted through the partial reflector 6 as shown by an arrow B. If the discharge causes the temperature of the gas to rise beyond a predetermined limit, the laser oscillation becomes difficult or impossible to maintain. Accordingly, the gas is circulated by the blower 9 in the direction of the arrows through ducts 7 and 8 and is cooled by a heat exchanger 10, whereby the gas temperature in the discharge tube 1 is maintained at a temperature lower than a predetermined value.
In such conventional apparatus the power source frequency must be at least 20 or 30 KHz in order to obtain the high discharge density necessary for performing efficient laser oscillation. Such high frequencies are disadvantageous, however, in that the conversion efficiency of the power source is reduced and the manufacturing costs are high.
Furthermore, with such a conventional arrangement the discharge density distribution in the tube 1 has a planar symmetry along the center plane C as shown in FIG. 2(a), which makes it difficult to obtain a high discharge density.
FIG. 2(b) shows a typical operating curve for a single phase discharge of the prior art; the relationship between the high frequency voltage V.sub.12 applied across the electrodes and the discharge period. As may be seen the discharge period is intermittent, which further reduces the overall discharge density.